In hydraulic drive systems for construction machines such as hydraulic excavators, widely used today are those equipped with a regulator for controlling the displacement (flow rate) of a hydraulic pump such that the delivery pressure of the hydraulic pump becomes higher by a target differential pressure than the maximum load pressure of a plurality of actuators. This type of control is called “load sensing control.” Such a hydraulic drive system for a construction machine equipped with a regulator for performing the load sensing control is described in Patent Document 1, in which a two-pump load sensing system including two hydraulic pumps each designed to perform the load sensing control is described.
The regulator of a hydraulic drive system for a construction machine performs torque control such that the absorption torque of a hydraulic pump does not exceed the rated output torque of the prime mover and prevents stoppage of the prime mover caused by excessive absorption torque (engine stall), generally by decreasing the displacement of the hydraulic pump as the delivery pressure of the hydraulic pump increases. In cases where the hydraulic drive system is equipped with two hydraulic pumps, the regulator of one hydraulic pump performs the torque control by taking in not only the delivery pressure of its own hydraulic pump but also a parameter regarding the absorption torque of the other hydraulic pump (total torque control) in order to prevent the stoppage of the prime mover and efficiently utilize the rated output torque of the prime mover.
For example, in Patent Document 2, the total torque control is performed by leading the delivery pressure of one hydraulic pump to the regulator of the other hydraulic pump via a pressure reducing valve. The set pressure of the pressure reducing valve is constant and has been set at a value simulating the maximum torque of the torque control performed by the regulator of the other hydraulic pump. With these features, in work in which only one or more actuators related to the one hydraulic pump are driven, the one hydraulic pump can efficiently use almost all of the rated output torque of the prime mover. Further, in work with a combined operation in which an actuator related to the other hydraulic pump is also driven at the same time, the total absorption torque of the pumps does not exceed the rated output torque of the prime mover and the stoppage of the prime mover can be prevented.
In Patent Document 3, in order to perform the total torque control on two hydraulic pumps of the variable displacement type, the tilting angle of the other hydraulic pump is detected as output pressure of a pressure reducing valve, and the output pressure is led to the regulator of the one hydraulic pump. In Patent Document 4, control precision of the total torque control is increased by detecting the arm length of a pivoting arm in place of the tilting angle of the other hydraulic pump.